Thermoplastic resin films, particularly biaxially drawn polyester films, have excellent properties such as mechanical properties, electrical properties, dimensional stability, transparency, and chemical resistance, and therefore have been widely used as a substrate film in a number of applications such as magnetic recording materials and packaging materials. Especially in recent years, they are in increasing demand as various optical films including display materials related to flat-panel display. In such a flat-panel display, a plurality of optical films having various functions are laminated for use in most cases. Therefore, methods for providing a polyester film surface with adhesion property have hitherto been studied. In particular, formation of an adhesion resin layer by coating provides adhesion to various materials.
However, there have been cases where, although adhesion property to various resins has been obtained, for example, heat treatment in processing has caused precipitation of oligomers from a thermoplastic resin film, resulting in unsuitability for practical use as an end product due to whitening or film surface contamination. Therefore, laminating a coating film on a resin film surface in order to inhibit oligomers has hitherto been studied. For example, the method of providing a coating film using an acrylic-modified polyester Japanese Patent Publication Nos. 04-263937 A, 2003-012841 A and 2002-011841 A, the method of adding an additive such as a resin having a particular functional group, mineral oil, or a cross-linker to a resin layer Japanese Patent Publication Nos. 2006-281498 A and 2002-127621 A, and the method of providing adhesion property and heat resistance property by laminating a coating film comprising various binder resins and a cross-linker using the in-line coating method in which application is carried out during the process of producing a thermoplastic resin film Japanese Patent Publication Nos. 2010-143202 A, 2006-321165 A and 2008-179148 Δ have been proposed.
However, according to the method of providing an acrylic-modified polyester on a film surface as a resin layer as described in Japanese Patent Publication Nos. 04-263937 A, 2003-012841 A and 2002-011841 A, although the acrylic-modified polyester contains an acrylic component having a glass-transition temperature of not less than a certain temperature, defects and cracks occur in the resin layer when the resin layer is set, resulting in that sufficient oligomer-inhibiting effect cannot be obtained, and besides the transparency of a laminated film can be impaired. In particular, when providing an acrylic resin having a glass-transition temperature of more than 90° C. as a resin layer as described in Japanese Patent Publication Nos. 2010-143202 A, 2006-321165 A and 2008-179148 A, cracks occur in the resin layer during film formation, and therefore haze can increase to significantly decrease homogeneous film-formation. Further, according to the method using an additive such as mineral oil or a cross-linker as described in Japanese Patent Publication Nos. 2006-281498 A and 2002-127621 A, the additive itself can bleed out on a surface layer of a resin layer during the resin layer formation or over time after the film formation, causing, for example, whitening of the resin film and film surface contamination as well as oligomer precipitation.
Thus, it could be helpful to provide a resin film having an excellent transparency and inhibition of oligomer.